Many types of electrical equipment require duplicated power systems to insure that power can be reliably and continuously delivered to a load. Typically, one power source will be designated the master power source for delivering AC or DC power to the load. In such a duplicated system, an auxiliary switched to the load on the detection of a failure of the master power source. The faulty power source is removed from the load so that is does not interfere with the online auxiliary power source. Many applications employing master and auxiliary power sources require that the switching from the faulty power source to the other power source occur in a very short period of time so that the load current or voltage is not substantially interrupted. Some situations, for example, submarine computerized navigation systems, require that power interruption during a switchover be no longer than a few milliseconds. Otherwise, the computerized system enters into a mode wherein normal operations cannot be sustained until several minutes thereafter.
The conventional approach in switching from one power source to another, either automatically or manually, is to connect the load to the master power source by a normally closed contact of a heavy duty relay or solenoid, and connect the load to the auxiliary power source through a normally opened contact of the relay. When the relay is operated, the switchover from the master power source to the auxiliary power source is accomplished. The problem attendant with this technique is that with large wattage power supplies which may range from several kilowatts to several thousand kilowatts, the relay contacts become deteriorated due to arcing, and thus becomes less reliable. The dangers of arcing are apparent in explosive atmospheres. Arc suppression circuits are well documented in the art, such as those disclosed in U.S. Pat. Nos. 4,251,845; 4,389,691 and 4,466,038.
An additional concern of critical importance exists when switching power sources generating AC currents. Preferably, power is switched from one power source to the other during zero-crossings of the alternating current cycle to reduce electrical arcing across the relay contacts. However, when the master and auxiliary power sources are not frequently synchronized, the zero-crossings thereof are also not synchronized, and thus the power sources cannot be simultaneously switched during respective low power portions of the AC cycle. This situation is aggravated when contact arc suppression devices, such as those disclosed in the noted patents, short circuit the relay contact before the actual opening and closing, and remain on a short period of time after the contact opening or closing. Thus, this presents a situation in which both contact may be momentarily closed, thereby short circuiting the master power source to the auxiliary power source.
From the foregoing, it can be seen that a need exists for a reliable and automatic bus transfer device which monitors various electrical parameters of each power source, and thus effects a transfer at the optimum time. An additional need exists for an improved automatic bus transfer device which can be easily applied to an existing relay-type bus transfer system, without modification of the system or degradation of the reliability thereof.